DC-to-DC converters for rapidly charging a capacitor from a low-level DC power supply are regularly used with cameras utilizing a gas discharge flash tube. Such converters must be highly reliable in addition to being small in size and low in cost. Additionally, they must be efficient and provide fast recycling time.
The electrical energy that must be stored in a capacitor prior to firing the flash tube depends on the parameters of the camera such as its lens and the film speed used with the camera, and on the maximum distance for flash photography, as well as the flash tube characteristics including reflector and tube designs. A relatively low value of electrical energy is five joules, and is typical for built-in strobes for many types of 35 mm cameras. For professional photography using larger cameras, 50 joules is a typical value for the electrical energy that must be stored in a capacitor. In converters of the type having an R-C controlled feedback oscillator, the oscillator runs at a predetermined frequency and its output is transformer-coupled to a rectifier that supplies current to the storage capacitor of the flash tube charging it to the desired level in a relatively short time. This technique is satisfactory for applications where battery power is readily available for driving the converter because feedback oscillation converters are inherently inefficient.
U.S. Pat. No. 3,541,420, granted Nov. 17, 1970, discloses one approach to improving the electrical efficiency of a DC-to-DC converter by providing a variable duty cycle driver circuit. In this patent, a transistor driver circuit is connected to the primary of a transformer across whose secondary is connected a capacitor to be charged, and the duty cycle of the transistor is controlled by a current sensor connected in the secondary of the transformer. The "on" time of the driver circuit is fixed for supplying a predetermined amount of energy to the magnetic field of the transformer. When the driver is turned off, the magnetic field collapses including current flow in the secondary of the transformer which is effective to transfer energy from the field to the capacitor. Current continues to flow in the secondary (i.e., the energy transfer process remains active) until the magnetic field collapses to zero. At that instant, secondary current flow terminates. The driver circuit remains "off" as long as the current sensor detects the presence of load current in the secondary. As soon as load current terminates (i.e., as soon as the energy transfer process is completed), a new cycle is initiated. In operation, the current sensor causes the duty cycle to vary inversely with the voltage on the capacitor; and the result is a converter having a slightly higher electrical efficiency than a conventional feedback oscillator type converter.
U.S. Pat. No. 4,104,714, granted Aug. 1, 1978, discloses a DC-to-DC converter that also minimizes transition losses. In this patent, operation of the driver circuit is controlled by the state of a flip-flop. A current sensor in the primary circuit controls the "on" time by sensing when the primary current reaches a predetermined limit below the saturation level of the transformer, and, in response, resetting the flip-flop to turn-off the driver transistor. The interruption of current in the primary causes the resultant magnetic field to begin to collapse thereby inducing a flow of current in the secondary which serves to charge the capacitor through a diode. The voltage across this diode is monitored and used to set the flip-flop when current flow through the diode terminates. The setting of the flip flop turns on the driver transistor and the cycle repeats.
In each of the two above identified patents, completion of the transfer of energy in the magnetic field of the transformer is sensed by determining the point at which the flow of current in the secondary reaches zero. Zero-point detection of current is difficult to achieve in practice because of the presence of noise; and as a consequence, some losses will be introduced in the event that the sensor arrangement utilized fails to detect the precise moment at which all of the energy contained in the magnetic field has been transferred to the capacitor. Generally, the losses involved are relatively small; but in an environment wherein the battery life is critical, the presence of such losses can result in marginal operation.
It is therefore an object of the present invention to provide a new and improved DC to DC converter wherein the above-referred to deficiencies in the prior art are substantially overcome or eliminated.